Color electrophoretic display

ABSTRACT

A color electrophoretic display has pixels which each comprise an image volume (IV) and a reservoir volume (RV). Different types of particles (Pf, Pm, Ps; Pa, Pb, Pc) which have different colors and different electrophoretic mobilities are present in each one of the pixels. The particles (Pf, Pm, Ps; Pa, Pb, Pc) which are present in the image volume (IV) determine a visible color of the pixel ( 10 ), and the particles (Pf, Pin, Ps; Pa, Pb, Pc) which are present in the reservoir volume (RV) do not contribute to the visible color of the pixel ( 10 ). The color electrophoretic display is driven to operate either in: a first mode wherein all the types of particles (Pf, Pin, Ps; Pa, Pb, Pc) contribute to a change of color of at least some of the pixels, or a second mode wherein only a subset of the types of particles (Pf, Pin, Ps; Pa, Pb, Pc) contribute to the change of the color of at least some of the pixels.

The invention relates to a color electrophoretic display, a method ofdriving a color electrophoretic display, and a display apparatuscomprising such a color electrophoretic display.

U.S. Pat. No. 6,271,823 discloses a reflective electrophoretic colordisplay. The display comprises pixel elements (also referred to aspixels) adjacently located in a plane. The pixels comprise at least twosub-pixels or cells which are also adjacently located in the same plane.The different cells of a pixel reflect a different color. The color of apixel is determined by the additive mixture of the colors reflected byeach of its respective cells.

Each cell comprises a light-transmissive front window, a non-obstructingcounter electrode, a light-reflective panel, a color filter medium, anda suspension of charged, light-absorbing pigment particles in alight-transmissive fluid.

The amount of colored light reflected by each cell is controlled by theposition of the pigment particles within the cell by applyingappropriate voltages to the collecting and counter electrodes. When thepigment particles are positioned in the path of the light, the light issignificantly attenuated before emerging from the front window, and theviewer sees a dim color or black. When the pigment particles aresubstantially removed form the path of the light, light can be reflectedback through the front window to the viewer without significantattenuation, and the viewer sees the color transmitted by the colorfilter medium. The color filter medium can, for example, be alight-transmissive colored filter element, a colored light-reflectingpanel, or the pigment suspension fluid itself.

It is an object of the invention to provide a color electrophoreticdisplay which has a higher refresh rate or lower power consumption whendisplaying display information which does not require use of all thedifferent colored pigment particles.

A first aspect of the invention provides an electrophoretic display asclaimed in claim 1. A second aspect of the invention provides a methodof driving an electrophoretic display as claimed in claim 14. A thirdaspect of the invention provides a display apparatus comprising such anelectrophoretic display as claimed in claim 16. Advantageous embodimentsof the invention are defined in the dependent claims.

In the color electrophoretic display in accordance with the first aspectof the invention the particles which have different colors havedifferent mobilities.

The color electrophoretic display comprises a driver which suppliesdrive voltages to the pixels to operate the color electrophoreticdisplay either in a first mode wherein all the types of particlescontribute to a change of color of at least some of the cells, or asecond mode wherein only a subset of the types of particles contributeto the change of the color of at least some of the cells. For example,in the first mode a full color image is displayed, and in the secondmode a monochrome image is displayed. Because in the second mode not allthe differently colored particles have to be moved to contribute to theimage displayed, the refresh rate can be increased, or at the samerefresh rate, the power consumption will decrease. The effect is maximalif only the fastest particles are used during the second mode.

The higher refresh rate is in particular relevant when monochrome videois displayed on a full color E-paper display which has in the full colormode a relatively low refresh rate.

In contrast, the prior art electrophoretic color display alwaysaddresses all of the sub-pixels of the pixels independent on the amountof colors required to display the image, and thus always uses all thedifferent colored pigment particles. The display of monochrome videowill show strong motion artifacts due to the low refresh rate.

In an embodiment in accordance with the invention as claimed in claim 2,the electrophoretic display has pixels which each comprise an imagevolume and reservoir volume. Each of the pixels is filled with differenttypes of particles having different colors and different electrophoreticmobilities. The particles determine a visible color of the pixel whenpresent in the image volume, the particles do not contribute to thevisible color of the pixel when present in the reservoir volume. Thecolor electrophoretic display further comprises a driver which suppliesdrive voltages to the pixels to operate the color electrophoreticdisplay either in a first mode wherein all the types of particlescontribute to a change of color of at least some of the cells, or asecond mode wherein only a subset of the types of particles contributeto the change of the color of at least some of the cells. Whichparticles are moved from the reservoir volume into the image volumedepends on the color a particular pixel should get in accordance with animage to be displayed. However, as there may exist pixels which requirea move of all types of particles into the image volume, all the types ofparticles have to be selected during a select period and for everyselected type of particle a fill period should be available to move theselected type of particles into the image area

In the first mode, all the different colored particles are selected inthe reservoir volume to be moved into the image volume. Which types ofparticles are actually moved into the image volume in which quantitydepends on the image to be displayed.

In the second mode, not all the different colored particles are selectedin the reservoir volume to be moved into the image volume because theimage has colors which allow using only a subset of the available typesof particles.

For example, in the first mode, when all the particle types areavailable to be moved into the image volume, a full color image can bedisplayed. Usually, it suffices to have three types of particles whichusually are colored magenta, yellow, and cyan. In the second mode, whenfor example, a monochrome image has to be displayed, it suffices toselect only one of the different types of particles to be available tobe moved into the image volume. As only one of the different types ofparticles has to be selected in the reservoir volume and only one fillperiod is required, either a higher refresh rate is possible in thesecond (monochrome video) display mode, or the power consumptiondecreases when the refresh rate is kept the same. Combinations of thesetwo effects are of course also possible.

U.S. Pat. No. 6,445,323 discloses a digital driver for a LCD display. Amode of operation of the digital driver is controlled in accordance withformat control signals. The different modes are: monochrome, color ofvarious resolutions, and a one bit superimpose function. The formatcontrol signals are used to optimize the picture quality and the powerconsumption. In the monochrome mode the drive signals are supplied toLCD cells of a single color only. However, U.S. Pat. No. 6,445,323B1, byits LCD nature wherein each color is associated with a LCD pixel, doesnot disclose how to proceed when a display comprises pixels which eachcontain different types of electrophoretic particles which havedifferent mobilities. Further, U.S. Pat. No. 6,445,323B1 does notdisclose how the different types of particles have to be selected in areservoir volume of the pixel and how these particles have to beselectively moved into the image volume of the pixel in accordance withthe color the pixel should get. A LCD is completely differentlycontrolled than an electrophoretic display, in a LCD display, the imagedisappears when the drive voltages are removed.

In an embodiment in accordance with the invention as claimed in claim 3,the driver adapts a refresh rate of the electrophoretic display duringthe second mode to obtain a display of the video information with asecond refresh rate being higher than the first refresh rate occurringduring the first mode. As explained earlier this allows improving thedisplay of moving display information if this moving display informationis displayed with colors allowing the use of a subset of the differenttypes of particles.

In an embodiment in accordance with the invention as claimed in claim 4,the pixel is constructed and driven to address the different types ofparticles sequentially. Each addressing phase comprises a select phaseand a fill phase. During each select phase one of the types of particlespresent in the reservoir volume is moved in front of the opening betweenthe reservoir volume and the image volume such that these particles canbe moved during the fill period into the image volume. The otherparticles are not in front of the opening and thus are obstructed to bemoved into the image volume during the fill period. The actual amount ofthe selected type of particles which are moved into the image volume ofa particular one of the pixels depends on the color this pixel shouldget in accordance with the image to be displayed.

Thus, during the first mode all the different types of particles have tobe sequentially addressed during an address cycle per pixel. The refreshrate of the display is determined by the number of pixels of the displaytimes the duration of the address cycle per pixel or per row of pixels.Usually, the pixels are selected row by row. Usually, the refresh ratefurther decreases due to a reset period which is required to reset allthe pixels to the same optical state before they are addressed.

During the second mode at least one of the different types of particlesneed not be addressed because the associated color is not required inthe image to be displayed. Thus, the total time to address the pixelswill become much shorter as at least one address cycle (a select periodand a fill period) less is required per pixel or row of pixels.Consequently, the refresh rate can be increased to better display video,or the power consumption will decrease because the drive of the displayis inactive during part of the time.

In an embodiment in accordance with the invention as claimed in claim 5,only a single one of the different types of particles is addressed. Thisallows displaying monochrome information at a higher refresh rate orwith lower power consumption.

In an embodiment in accordance with the invention as claimed in claim 6,only the type of particles is addressed which has the highest mobility.This minimizes the time required for addressing the pixels, for movingthe particles from the reservoir volume into the image volume, and forresetting the particles from by moving them back to the reservoirvolume.

In an embodiment in accordance with the invention as claimed in claim 7,select electrodes are present which generate in the reservoir volume aselect electric field which separates the different types of particlesin different sub-volumes in the reservoir volume. A voltage suppliedbetween the select electrodes generates a select electric field whichexerts a force on the particles. The particles will start moving due tothis force with a speed which depends on the mobility of the particles.Within a particular time period that the select electric field ispresent, particles with a high mobility will move further than particleswith a low mobility. In this manner, it is possible to separate thedifferent particles in different sub-volumes of the reservoir volume.

Fill electrodes generate a fill electric field to move the differenttypes of particles from the different sub-volumes into the image volume.The fill electric field moves the particles which are separated in thedifferent sub-volumes into the image volume to determine the color ofthe pixel. The color of the pixel will depend on the time period thefill electric field is present. If the fill electric field is presentfor a short duration, much more particles with the highest mobility willbe moved into the image volume than the particles with the lowestmobility. If the fill electric field is present for a long duration, allthe particles will be moved into the image volume and thus differentcolors of the pixel are possible with a single image volume. It is notrequired to have several separate cells to obtain different colors.Consequently, if the image volume is equal to the volume of a prior artcell, the pixel in accordance with the invention will cover a smallerarea and thus the resolution of the display can be higher. If the pixelvolume of the pixel in accordance with the invention is equal to thevolume of the several cells of a prior art pixel, the brightness maybecome higher, as the pixel boundaries occupy less pixel volume or area.Since the portion of each prior art pixel producing the desired color issmaller than in the present invention, the color will appear much lessbright than if the entire pixel were able to produce the required coloras is the case in the present invention.

Although the display in accordance with the invention as defined inclaim 7 is able to provide different colors, it is not possible to makeany possible combination of color shades of the different colors of thedifferent particles.

In an embodiment as defined in claim 8, the at least one fill electrodeis positioned to obtain a fill electric field directed to simultaneouslymove the different types of particles from the sub-volumes into theimage volume. This has the advantage that the time required to fill theimage volume with the particles decreases considerably.

In an embodiment as defined in claim 9, the fill electric field can becontrolled for each type of particle separately, and thus, the number ofparticles of each type which are transported from the sub-volumes to theimage volume can be freely controlled. Consequently, it is possible tomake all color shades based on the different colors of the differentparticles. If not all the different types of particles are required toproduce the image, only a subset need to be moved into the image volume.The select period may become shorter as it suffices that only the typesof particles which may have to be moved into the image volume are movedin the reservoir volume until they can be moved into the image volume. Afaster addressing and thus a higher refresh rate is possible, already ifonly the slowest type of particles is not used.

In an embodiment as claimed in claim 10, the pixel comprises a furtherreservoir volume. The pixel comprises further select electrodes and fillelectrodes which are associated with the further reservoir in the samemanner as the first mentioned select electrodes and the first mentionedfill electrodes are associated with the first mentioned reservoirvolume. The function of the further reservoir volume is the same as thefirst mentioned reservoir volume. This embodiment has the advantage thatthe refresh rate of the display can be increased further because theselection process in one of the reservoirs can be performed in parallelwith the filling or reset process from another reservoir as defined inclaim 10. It is possible to associate more than two reservoirs with asame image volume.

In an embodiment as claimed in claim 12, the pixel comprises a furtherfill electrode which is positioned to enlarge the fill electric field inthe image volume to speed up the filling of the visible part of thepixel by particles entering the image volume from the sub-volumes.

In an embodiment as claimed in claim 13, the distance of the furtherfill electrode to the sub-volumes varies such that the further fillelectrode is nearest to the store volume in the reservoir volume. Thishas the advantage that a higher field is obtained for the particles suchthat the speed of movement of the particles increases and the fillingtime of the image volume decreases.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a construction of a pixel of an electrophoretic display,

FIG. 2 shows waveforms for operating the pixel shown in FIG. 1 in a fullcolor electrophoretic display,

FIG. 3 shows another construction of a pixel of an electrophoreticdisplay,

FIG. 4 shows another construction of a pixel of an electrophoreticdisplay,

FIG. 5 shows another construction of a pixel of an electrophoreticdisplay, and

FIG. 6 shows a block diagram of a display apparatus with anelectrophoretic matrix display of an embodiment in accordance with theinvention.

FIG. 1 shows a construction of a pixel of an electrophoretic display.The pixel volume comprises a reservoir volume RV and an image volume IV.Three different types of particles Pf, Pm, Ps are present which havedifferent colors and different mobilities. As elucidated with respect toFIG. 2, during a select period, the different types of particles Pf, Pm,Ps have to be selected in the reservoir volume RV one by one to be movedto the opening OP between the reservoir volume RV and the image volumeIV. The particles Pf, Pm, Ps are moved by applying a select electricfield SF in the reservoir volume RV. The rest of the reservoir volume RVand the image volume IV are separated by the rib RI. During a fillperiod, a fill electric field FF moves the particles present at theopening into the image volume Iv of the pixel, dependent on the color tobe displayed. The select electrodes E1 and E2 are positioned withrespect to the reservoir volume RV to be able to move the particleswhich, initially are attracted to the select electrode E1, towards theopening OP. The fill electrodes E3 and E4 are positioned with respect tothe image volume IV to move the selected particles which are near theopening OP into the image volume IV during the fill period, or to movethe particles which are in the image volume IV back into the reservoirvolume during a reset period. The operation of the pixel is elucidatedin more detail with respect to FIG. 2.

FIG. 2 shows waveforms for operating the pixel shown in FIG. 1 in a fullcolor electrophoretic display.

First is elucidated how the electrophoretic display is operated in thefirst mode wherein polychrome information is displayed and all the typesof particles may contribute to a change of color of the cells.

In a first step, a reset pulse RE1 is supplied to the select electrodeE1 to gather all the particles Pf, Pm, Ps near the select electrode E1.If the particles Pf, Pm, Ps are negatively charged, the reset pulse REshould be positive. Next a voltage pulse SE1 is supplied between theselect electrodes E1 and E2 such that the select electrode E2 ispositive with respect to the select electrode E1 and the all theparticles Pf, Pm, Ps are attracted towards the select electrode E2. Whenthe fastest particles Pf (for example the cyan colored particles) arriveat the opening OP near the select electrode E2, the voltage pulse SE1 onthe select electrode E2 is switched off. The other slower particle typeshave not yet arrived at the opening OP. Then, the fastest particles Pfcan be drawn into the image volume IV of the pixel by means of theelectric field generated by the fill pulse FP1 on the fill electrodes E3and E4. The other particles Pm and Ps will not be drawn into the imagevolume IV by the electric field generated by the fill electrodes E3 andE4 because they are obstructed by the rib RI.

In a second step, a second reset pulse RE2 is supplied to the selectelectrode E1 to gather all the particles Pf, Pm, Ps near the selectelectrode E1. Then, a voltage pulse SE2 is supplied to the selectelectrode E2 during a longer period in time required to move both thefastest particles Pf and the particles Pm with the medium mobility tothe opening OP. Now a short repulsive pulse RP1 is supplied to theselect electrode E2, or a short attractive pulse RP1 is supplied to theselect electrode E1 to move the fastest particles Pf (for examplecolored cyan) back towards the direction of the electrode E1. Theparticles Pm with the medium mobility (for example colored magenta) havehardly had time to move away from the opening O2 so that they can bedrawn into the image volume IV by an appropriate voltage pulse FP2 onthe fill electrodes E3 and E4 during the fill period.

The last step, is to address the slowest particles Ps (for examplecolored yellow). First, the select electrode E2 receives a voltage pulseRE3 for a third reset wherein all the particles Pf, Pm, Ps are gatherednear the select electrode E2. Then, a voltage pulse SE3 is supplied tothe select electrode E1 to move the two fastest kinds of particles (cyanand magenta) away from the select electrode E2 in the direction of theselect electrode E1, whereas the slowest yellow particles Ps remain nearto the select electrode E2 and thus near to the opening OP. A voltagepulse FP3 on the fill electrodes E3 and E4 will move these yellowparticles Ps into the image volume IV during the fill period.

Thus, to be able to operate the electrophoretic display in the firstmode wherein polychrome information is displayed, all the particles Pf,Pm, Ps have to be sequentially selected in the reservoir volume RV andmoved into the image volume IV in accordance with the color to bedisplayed. All these sequential steps have to be performed before a nextcolor in accordance with the polychrome information can be displayed bythe same pixel or cell. A refresh time of the electrophoretic display isthus limited by the time required to perform these three sequentialsteps.

The electrophoretic display is operated in a second mode whereininformation is displayed with a reduced amount of colors and thus notall the types of particles are required. Now, less of steps have to beperformed than with respect to the display of polychrome informationwherein all the types of particles have to be used.

In the special situation that monochrome information has to displayed itsuffices to use a single type of the particles. It is only required toselect a single type of particles and to move these particles into theimage volume IV in accordance with the monochrome information to bedisplayed. Preferably, only the fastest particles are selected to bemoved into the image volume IV. The refresh time will become muchshorter as only one type of particles has to be selected and moved intothe image volume IV. Thus, the monochrome information is displayed witha higher refresh rate than the polychrome information. This minimizesflicker artifacts which are particular disturbing when reading largeamounts (of non-moving) text. The increased rate of update of imagesreduces the blurring of moving images. Alternatively, it is possible tokeep the refresh rate unaltered to obtain lower power consumption.

FIG. 3 shows another construction of a pixel of an electrophoreticdisplay. The pixel has a pixel volume PV which comprises a reservoirvolume RV and an image volume IV. In the pixel, three differentlycolored particles Pa, Pb, Pc with a different electrophoretic mobilityare present. The visible color of the pixel is determined by the amountof the particles Pa, Pb, Pc which is present in the image volume IV.Preferably, the colors of the particles are selected to be able toproduce a maximum amount of hues. For example, the particles are coloredyellow, magenta and cyan. The select electrodes SE1 and SE2 are presentat opposite sides of the reservoir volume RV to generate a selectelectric field SF (further also referred to as select field SF) in thereservoir volume RV in the y-direction. The fill electrodes FE1 and FE2are present in a plane which is perpendicular to the plane in which theselect electrodes SE1 and SE2 are present. The fill electrodes FE1 andFE2 generate a fill electric field FF (further also referred to as fillfield FF) in the x-direction perpendicular to the y-direction.

In general, all electrodes can be formed as thin conducting layerssituated on one of the substrate layers of which the cell is comprised.The electrodes, and in particular the fill electrode FE2 may also be inthe form of barriers, having many small holes or a few large holes toallow the particles Pa, Pb, Pc to pass, or the fill electrode FE2 maycomprise at least one strip.

To enable a rendering of different polychrome pictures on the display,the pixel is driven as elucidated in the following description.

At the start of a display period (also referred to as refresh period) ofthe pixel wherein the color of the pixel has to be adapted inconformance with the data to be displayed during this display period,during a reset phase, all colored particles Pa, Pb, Pc which were movedinto the image volume IV in accordance with previous image data areremoved from the image volume IV into the store volume SV of thereservoir volume RV by using an attractive voltage pulse on the selectelectrode SE1 to generate an electric field RF. Thus, in an initialstate, the colored particles Pa, Pb, Pc are stored in the store volumeSV such that all the particles Pa, Pb, Pc have a substantially samestarting position.

During the select phase, the particles Pa, Pb, Pc are separated withinthe reservoir volume RV using an attractive voltage pulse between theselect electrodes SE1 and SE2 to attract the particles Pa, Pb, Pctowards the select electrode SE2. The most mobile particles Pc move thefarthest, the particles Pa with the lowest mobility move over thesmallest distance, the particles Pb with an in-between mobility moveover a distance in-between the other distances. Thus, after the voltagepulse has been present between the select electrodes SE1 and SE2 duringa suitable duration, the particles Pa, Pb, Pc are separated: theparticles Pa are substantially present in the sub-volume SVa, theparticles Pb are substantially present in the sub-volume SVb, and theparticles Pc are substantially present in the sub-volume SVc, as isshown in FIG. 3. The sub-volumes SVa, SVb, SVc are schematicallyindicated by ellipsoids.

During the fill phase, all particles Pa, Pb, Pc are moved simultaneouslyfrom the sub-volumes SVa, SVb, SVc of the reservoir volume RV to theimage volume IV using an attractive voltage pulse between the fillelectrodes FE1 and FE2. As soon as sufficient particles Pa, Pb, Pc haveentered the pixel volume PV, the attractive voltage pulse is removedfrom the fill electrodes FE1 and FE2.

As the particles Pa, Pb, Pc are moved simultaneously from the reservoirvolume RV to the image volume IV, the refresh time of the pixel can bekept quite short. Once the particles Pa, Pb, Pc are within the imagevolume IV, they will be held there by a small repulsive voltage on thefill electrode FE2 until the next refresh period. During this image holdtime, the particles Pa, Pb, Pc can mix by Brownian motion, or, whenneeded, (AC) electrical signals can be used to effectuate particlemixing inside the pixel.

Preferably, as shown, the fill electrode FE2 comprises three sub fillelectrodes FE2 a, FE2 b, FE2 c to generate a fill field which has threesub-fill fields FFa, FFb, FFc in the sub-volumes SVa, SVb, SVc,respectively. Thus now, three different (in strength and/or duration)fill electric fields FFa, FFb, FFc may be present, allowing toseparately control the amount of particles Pa, Pb, Pc which will bemoved into the image volume IV.

Preferably, the fill electrode FE1 comprises arms FE1 a and FE1 b whichextend in the x-direction. These arms FE1 a and FE1 b shield the fillfields FFa, FFb, FFc occurring in adjacent ones of the sub-volumes SVa,SVb, SVc from each other. This reduces cross-talk effects in controllingthe amount of particles Pa, Pb, Pc which have to leave the sub-volumesSVa, SVb, SVc. In a preferred embodiment, FE1 a and FE1 b areimplemented as separate electrodes which may have individually definablevoltages. This further increases the efficiency of selecting particlesand filling the image volume.

A further fill electrode CF may be present to speed up the filling ofthe image volume IV by generating a further fill field FFF in the imagevolume IV to attract the particles Pa, Pb, Pc further into the imagevolume IV.

As soon as sufficient particles Pa, Pb, Pc have entered the image volumeIV (i.e passed the smaller fill electrodes FE2 a, FE2 b, FE2 c) excessparticles Pa, Pb, Pc may be sent back using these smaller pixelelectrodes FE2 a, FE2 b, FE2 c.

The arrow RF indicates the electric field required to the move theparticles Pa, Pb, Pc into the store volume SV during the reset phase ofthe pixel when a high voltage is present on the select electrode SE1.The display may be constructed such that a high voltage can be supplieddirectly to the select electrode SE1 to speed up the reset phase. If thevoltage has to be supplied to the select electrodes via TFT's, thevoltage level will be limited.

It is also possible to add a reset electrode, for example in the imagevolume IV, to increase the field which directs the particles Pa, Pb, Pcback into the reservoir RE. Preferably this extra reset electrode ispositioned in the center of the image volume IV. During the reset phase,first a voltage is supplied to the extra reset electrode to concentratethe particles Pa, Pb, Pc in the center of the pixel and then, a voltageis supplied to the select electrode SE1 to attract the particles Pa, Pb,Pc into the store volume SV. Alternatively, one of the existingelectrodes, for example FE2 a, may temporarily take the function of anadditional reset electrode during the reset phase.

In the geometry of the reservoir volume RV shown in FIG. 3, the mobilityof the slowest particle Pa is typically three times lower than that ofthe fastest particle Pc. It is possible to change the geometry of thereservoir volume RV such that a distance from the store volume SV to thesub-volumes becomes much larger. Due to the long reservoir, theparticles Pa, Pb, Pc can be separated even if the difference in themobility is far smaller. For example, the mobility of the slowestparticle Pa can be selected to be 75% of the mobility of the fastestparticle Pc. Consequently, as the mobility of the slowest particle Pa ismuch higher, the time required to fill the image volume IV and the timeto move the particles Pa, Pb, Pc back into the store volume SV decreasesconsiderably.

In the second mode wherein the electrophoretic display is operated todisplay monochrome information, the drive of the electrophoretic displayis adapted such that only the particles Pf with the highest mobility areselected to be moved into the image volume IV. This is realized byapplying the voltage between the select electrodes SE1 and SE2 during ashorter time than in the polychrome mode such that the fastest particlesPf are moved into the sub-volume SVa, while the other, slower, particlesPm and Ps are still in the store volume SV. The fastest particles Pf inthe sub-volume SVa are then moved into the image volume IV. As only thefastest particles Pf need to be moved into the image volume IV, also theduration of the fill period will be shorter than in the polychrome mode.

It is also possible to use the particles with the highest and with themedium mobility instead all the particle types. Still, the total timerequired to select and move these two type of particles into the imagevolume IV is shorter than when the electrophoretic display is operatedin the polychrome mode wherein all the types of particles, thus also theslowest, have to be selected and moved. Thus, it is possible to displaythe information which does not require all the types of particles to bemoved into the image volume IV with a higher refresh rate than thepolychrome information, or to decrease the power consumption. The gainis largest when monochrome information is displayed by using only thefastest particles.

FIG. 4 shows another construction of a pixel of an electrophoreticdisplay.

The pixel shown in FIG. 4 is based or the pixel shown in FIG. 3 whereinthe further fill electrode CF is removed and a second reservoir FRV isadded positioned opposite to the reservoir RV. The construction of thereservoir FRV may be identical to the construction of the reservoir RV.

Because the pixel should be constructed to allow display of polychromeinformation, the construction of the pixel which is able to display afull color picture is discussed. In such a pixel at least threeparticles should be present having primary colors.

The extra reservoir FRV comprises: the select electrodes SEV1 and SEV2,three sub-fill electrodes FFE2 a, FFE2 b, FFE2 c to generate thesub-fill fields FFFa, FFFb, FFFc in the sub-volumes FSVa, FSVb, FSVc,respectively. Thus again, three different (in strength and/or duration)fill electric fields FFFa, FFFb, FFFc may be present, allowing toseparately control the amount of particles FPa, FPb, FPc which will bemoved from the reservoir volume FRV into the image volume IV. In thiscase, sub-fill electrodes FE2 a, FE2 b, FE2 c can temporarily take therole of the further fill electrode CF to speed up the filling of theimage volume IV by generating a further fill field FFF in the imagevolume IV to attract the particles further into the image volume IV.

The fill electrode FEV1 comprises arms FFE1 b and FFE1 a which extend inthe x-direction. These arms FFE1 a and FFE1 b shield the fill fieldsFFFa, FFFb, FFFc occurring in adjacent ones of the sub-volumes FSVa,FSVb, FSVc from each other. This reduces cross-talk effects incontrolling the amount of particles FPa, FPb, FPc which have to leavethe sub-volumes FSVa, FSVb, FSVc.

During the reset period of the extra reservoir volume FRV, the particlesFPa, FPb, FPc are attracted by the store field FRF into the store volumeFSV.

The arrows indicated by aF, bF, cF show the movement of the particlesFPa, FPb, FPc, respectively, during the fill phase of the image volumeIV from the reservoir FRV.

The embodiment in accordance with the invention as shown in FIG. 3 hasthe drawback that after removing the particles from the pixel volume PVduring the reset phase, it is first necessary to select the particlesPa, Pb, Pc before the image volume IV can be filled.

In the preferred embodiment as shown in FIG. 4, the image volume IV willbe in contact with two reservoir volumes SV and FSV, whereby theparticles FPa, FPb, FPc are reset into the store volume FSV of thereservoir volume FRV, and the particles Pa, Pb, Pc are selected in theother reservoir volume RV. In this manner, the separation of theparticles Pa, Pb, Pc (the color selection) can be carried out prior tothe start of the refresh period of the other reservoir volume FRV. It isthen possible to move directly from the reset phase for the reservoirvolume FRV to the fill phase from the reservoir RV, thereby furtherreducing the refresh time.

This is also useful to further increase the refresh rate in themonochrome mode wherein only the fastest particles Pf are used to fillthe image volume IV.

The optional fill electrode CF is positioned slanted with respect to thereservoir RV such that the distance to the particles Pa, FPa in thesub-volume SVa, FSVa, respectively, is shorter than the distance to theparticles Pc, FPc in the sub-volume SVc, FSVc, respectively. Thedimensions of the image volume IV are the same. In this construction,the electrical field for pulling the particles out of the sub-volumesSVa or FSVa is larger. This is advantageous in the polychrome modewherein all the types of particles are used to speed up the movement ofthe slowest particles Ps and also during the monochrome mode (or a modewherein not all the types of particles are used) to speed up themovement of the fastest particles Pf (or the types of particles used).Thus again, the refresh rate can be further increased.

FIG. 5 shows another construction of a pixel of an electrophoreticdisplay. Now, each pixel comprises three sub-pixels. Each sub-pixelcontains a different types of particles dissolved in a solventcontaining a black dye. Particles near the top electrode are visible tothe observer. The fastest particles Pf are present in display cell CE1,the slowest particles Ps are present in the display cell CE3, and theparticles with the intermediate mobility are present in the display cellCE2.

FIG. 5A shows the full color operation wherein all the different typesof particles may have to be moved dependent on the color in accordancewith the image to be displayed the pixel should have. In FIG. 5B onlythe fastest particles Pf are used, the other particle types remain setto their black state. Although it is possible to display a monochromeimage only, the refresh rate can be increased significantly, as theslower particles do not hamper the speed of operation of theelectrophoretic display.

More general, a higher refresh rate is possible as soon as the slowestparticle is not used and thus no address cycle for this particle isrequired.

FIG. 6 shows a block diagram of a display apparatus with anelectrophoretic matrix display of an embodiment in accordance with theinvention. The display 1 comprises a matrix of pixels 10 atintersections of crossings row or selection electrodes 7 and column ordata electrodes 6. Two select electrodes SE1, SE2 and four dataelectrodes FE1, FE2 a, FE2 b, FE2 c correspond to one pixel 10. Theselect electrodes SE1 may be interconnected. The data electrodes FE1 mayalso be interconnected.

The rows 1 to m of the pixels 10 are consecutively selected by means ofa row driver 4, while the groups of column electrodes 1 to n areprovided with data via a data register 5. Each pixel 10 comprises areservoir volume RV and an image volume IV. A full color pixel 10comprises only a single image volume IV.

The incoming data 2 are first processed, if necessary, in a dataprocessor 3. Mutual synchronization between the row driver 4 and thedata register 5 takes place via drive lines 8.

Drive signals from the row driver 4 are supplied to the selectelectrodes SE1 and SE2 to separate the particles Pa, Pb, Pc in thesub-volumes SVa, SVb, SVc during the select period, and to move theparticles Pa, Pb, Pc back into the store volume SV during the resetphase.

Drive signals from the data driver 5 are supplied to the fill electrodesFE1, FE2 a, FE2 b, FE2 c to move the separated particles Pa, Pb, Pc fromthe reservoir volume RV into the image volume IV. The voltage on theextra fill electrode CF, when present, may also be supplied by the datadriver 5.

Such driving may be suitable for small matrix or segmented displays.More generally however, the display will be driven by an active matrix,comprising thin film transistors (TFTs), diodes or other activeelements. In the case of a TFT active matrix, each pixel will furthercomprise a multiplicity of addressing (or selection) TFTs. A line ofpixels is selected by applying a pulsed voltage to the addressing TFTs,whereby these become conductive and connect the electrodes in the pixelto data signals being generated by the data driver 5. It is alsopossible that some electrodes are common to a multiplicity of pixels.

The known drive is easily adapted to cater for the use of less than alltypes of particles. In the sequentially driven display of FIGS. 1 and 2,the sequence of voltages for the type of particles not used is left out.In the parallel driven display of FIGS. 3 to 4, only the fastest typesof particles are used. The selection of the particles is performed byusing a shorter select time such that only the particles to be movedinto the image volume IV are moved out of the store volume SV. Also thefilling and resetting is performed during shorter periods of time, as atleast the slowest particles are not anymore used. In the display inwhich a pixel comprises three sub-pixels, the drive is adapted toaddress one of the sub-pixels only.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

For example, it is not essential to the invention that three differenttypes of particles are present, what matters is that different types ofparticles are present. In the sequential addressed display, theadvantages of a higher refresh time or less dissipation are reached ifless than all the particle types are selected. In the parallel addresseddisplay the advantages are reached if at least one of the particle typeswhich does not have the lowest mobility to display information isselected. The particles may be positively charged instead of negatively.It is also possible to combine positively and negatively chargedparticles.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” does notexclude the presence of other elements or steps than those listed in aclaim. The invention can be implemented by means of hardware comprisingseveral distinct elements, and by means of a suitably programmedcomputer. In the device claim enumerating several means, several ofthese means can be embodied by one and the same item of hardware.

1. A color electrophoretic display comprising: pixels each comprisingdifferent types of particles (Pf, Pm, Ps; Pa, Pb, Pc) having differentcolors and different electrophoretic mobilities, and a driver (4, 5) forsupplying drive voltages to the pixels to operate the colorelectrophoretic display either in: a first mode wherein all the types ofparticles (Pf, Pm, Ps; Pa, Pb, Pc) contribute to a change of color of atleast some of the pixels, or a second mode wherein only a subset of thetypes of particles (Pf, Pm, Ps; Pa, Pb, Pc) contribute to the change ofthe color of at least some of the pixels.
 2. A color electrophoreticdisplay as claimed in claim 1, wherein the pixels each comprise an imagevolume (IV) and a reservoir volume (RV), and wherein the different typesof particles (Pf, Pm, Ps; Pa, Pb, Pc) determine a visible color of thepixel (10) when present in the image volume (IV), and wherein theparticles (Pf, Pm, Ps; Pa, Pb, Pc) do not contribute to the visiblecolor of the pixel (10) when present in the reservoir volume (RV).
 3. Acolor electrophoretic display as claimed in claim 1, wherein the driver(4, 5) comprises means (4, 5) for adapting a refresh rate of theelectrophoretic display during the second mode to obtain a display ofthe video information with a second refresh rate being higher than thefirst refresh rate occurring during the first mode.
 4. A colorelectrophoretic display as claimed in claim 2, wherein the reservoirvolume (RV) comprises select electrodes (E1, E2) for generating a selectelectric field (SF) in the reservoir volume (RV), wherein the imagevolume (IV) comprises fill electrodes (E3, E4) for generating a fillelectric field (FF) in the image volume (IV), the select electric field(SF) extending in a first direction (y), the fill electric field (FF)extending in a second direction (x) not being aligned with the firstdirection (y), and wherein the particles (Pf, Pm, Ps) are able to movefrom the reservoir volume (RV) to the image volume (IV) only locallyalong a distance between the select electrodes (E1, E2), the driver (4,5) being adapted to supply voltage pulses to the select electrodes (E1,E2) and the fill electrodes (E3, E4) to move the different groups ofparticles (Pf, Pm, Ps) sequentially into the image volume (IV).
 5. Acolor electrophoretic display as claimed in claim 4, wherein the driveris adapted for selecting only a single one of the different types ofparticles (Pf, Pm, Ps) during the second mode, and to move theseparticles (Pf, Pm, Ps) into the image volume (IV) in accordance with amonochrome image to be displayed.
 6. A color electrophoretic display asclaimed in claim 5, wherein the particles (Pf, Pm, Ps) of the single oneof the different types of particles are the particles (Pf) having thehighest mobility.
 7. A color electrophoretic display as claimed in claim2, further comprising select electrodes (SE1, SE2) for generating in thereservoir volume (RV) a select electric field (SF) for separating thedifferent types of particles (Pa, Pb, Pc) in different sub-volumes (SVa,SVb, SVc) in the reservoir volume (RV), and at least one fill electrode(FE1, FE2) for generating a fill electric field (FF) to move thedifferent types of particles (Pa, Pb, Pc) from the sub-volumes (SVa,SVb, SVc) into the image volume (IV).
 8. An electrophoretic display asclaimed in claim 7, wherein the at least one fill electrode (FE1, FE2)is positioned to obtain the fill electric field (FF) directed forsimultaneously moving the different types of particles (Pa, Pb, Pc) fromthe sub-volumes (SVa, SVb, SVc) into the image volume (IV).
 9. Anelectrophoretic display as claimed in claim 7, wherein the fillelectrodes (FE2) comprise sub fill electrodes (FE2 a, FE2 b, FE2 c)associated with the different sub-volumes (SVa, SVb, SVc) for generatingthe fill electric field (FF) to comprise sub fill electric fields (FFa,FFb, FFc) in the different sub-volumes (SVa, SVb, SVc).
 10. Anelectrophoretic display as claimed in claim 7, further comprising: afurther reservoir volume (FRV), further select electrodes (SEV1, SEV2)for generating in the further reservoir volume (FRV) a further selectelectric field (SFV) for separating the different types of particles(FPa, FPb, FPc) in further different sub-volumes (FSVa, FSVb, FSVc) inthe further reservoir volume (FRV), and further fill electrodes (FFE2 a,FFE2 b, FFE2 c) for generating a further fill electric field (FFFa,FFFb, FFFc) to simultaneously or time sequentially move the differenttypes of particles (FPa, FPb, FPc) from the further sub-volumes (FSVa,FSVb, FSVc) into the image volume (IV).
 11. An electrophoretic displayas claimed in claim 7, wherein the electrophoretic display comprises acontroller for controlling the first mentioned select electrodes (SE1,SE2), the at least one first mentioned fill electrode (FE1, FE2), thefurther select electrodes (SEV1, SEV2), and the further fill electrodes(FFE2 a, FFE2 b, FFE2 c) to obtain a separation of the different typesof particles (Pa, Pb, Pc) in the first mentioned reservoir volume (RV)simultaneously to filling or resetting particles (FPa, FPb, FPc) to orfrom the further reservoir volume (FRV), or the other way around.
 12. Anelectrophoretic display as claimed in claim 11, wherein the pixel (10)comprises a further fill electrode (CF) arranged in the image volume(IV) in the second direction further away from the reservoir volume (RV)than the sub fill electrodes (FE2 a, FE2 b, FE2 c) for attracting theparticles (Pa, Pb, Pc) leaving the sub-volumes (SVa, SVb, SVc) furtherinto the image volume (IV).
 13. An electrophoretic display (1) asclaimed in claim 12, wherein the further fill electrode (CF) ispositioned with respect to the sub-volumes (SVa, SVb, SVc) to obtain asmallest distance towards the sub-volume (SVa) nearest to a store volume(SV) in the reservoir volume (RV).
 14. A method of driving a colorelectrophoretic display having pixels comprising different types ofparticles (Pf, Pm, Ps; Pa, Pb, Pc) having different colors and differentelectrophoretic mobilities, the method comprising supplying (4, 5) drivevoltages to the pixels to operate the color electrophoretic displayeither in: a first mode wherein all the types of particles (Pf, Pm, Ps;Pa, Pb, Pc) contribute to a change of color of at least some of thepixels, or a second mode wherein only a subset of the types of particles(Pf, Pm, Ps; Pa, Pb, Pc) contribute to the change of the color of atleast some of the pixels.
 15. A method as claimed in claim 14, whereinthe pixels each comprise an image volume (IV) and a reservoir volume(RV), and wherein the particles (Pf, Pm, Ps; Pa, Pb, Pc) determine avisible color of the pixel (10) when present in the image volume (IV),and wherein the particles (Pf, Pm, Ps; Pa, Pb, Pc) do not contribute tothe visible color of the pixel (10) when present in the reservoir volume(RV).
 16. A display apparatus comprising a color electrophoretic displayas claimed in claim 1.